Firearms having a direct gas impingement system or an indirect gas impingement system are known. Direct gas impingement is a type of gas operation for a firearm that directs gas from a fired cartridge directly to the bolt carrier or slide assembly to cycle the action in the firearm.
More specifically, in a direct gas impingement system, when the firearm is fired, the exhaust propellant gases from the fired cartridge are directed through a port at the end of the barrel and then channeled back to the bolt carrier and will strike, or impinge, the bolt carrier moving it rearward toward the buttstock and into a retracted position. The exhaust gases will then discharge out the ejection port on the side of the firearm near the buttstock. After discharge, the spring acting on the bolt carrier will move the bolt carrier back to the engaged position at the same time causing the bolt to pick up another cartridge from the magazine and move that cartridge into a battery position within the firearm's breech.
There are several known disadvantages with a direct gas impingement system. As an example, one disadvantage is that the breech of the firearm becomes fouled more quickly. This is caused by solids and impurities from the high-temperature gas from the fired cartridge condensing as they cool and being deposited on the bolt face and primary operating mechanism. Thorough and frequent cleaning is required to ensure reliability and proper operation of the firearm's operating mechanism. The amount of fouling depends upon the firearm's design as well as the type of propellant powder used in the fired cartridge. A further disadvantage is that combustion gases from the fired cartridge heat the bolt and bolt carrier as the firearm operates. This heating may alter the temper of metal parts, accelerating wear and decreasing the service life of the bolt, extractor, and extractor spring. Additionally, heat dries up the firearm's lubricant and makes the firearm's operating parts difficult to handle when clearing malfunctions. Heat can also melt the lacquer coatings of steel cartridge cases, gumming up parts. Moreover, thermal expansion in the firearm's action can result in loss of tolerances and consequent degradation in the firearm's accuracy.
Firearms having an indirect gas impingement system differ from the direct gas impingement system in that the exhaust gases do not directly act on the bolt carrier. Rather, the exhaust gases, after the firearm has been fired, act on and move a piston-type rod that, in turn, is operatively connected to the bolt carrier. The movement of the piston-type rod moves the bolt carrier rearward, or in the direction opposite to the fired bullet, and to a retracted position. Once the piston has traveled a certain distance, the remaining unused gas acting on the piston-type rod is discharged through a port on the firearm. A spring acting on the piston will then move the rod and accompanying bolt and bolt carrier forward, picking up a new cartridge, and moving that cartridge into the battery position.
It is also known that a firearm may be modified to provide full auto firing capability. To accomplish this, it is known to use a drop-in auto sear. When used, and when the operator pulls the trigger, the drop-in auto sear intercepts the hammer before the disconnector intercepts the hammer (i.e., bypasses the disconnector). The auto sear holds the hammer and functions like the disconnector until the bolt and bolt carrier move forward into the battery position. Typically, the bottom rear portion of the bolt carrier that extends down contacts the auto-sear which releases the hammer. The bolt and bolt carrier are fully back into battery position just before the hammer hits the firing pin, which causes the firearm to discharge a round. The cycle continues until the operator releases the trigger.
More specifically, in a normal, semi-automatic operation, the trigger's front acts as the sear. When the hammer is cocked there is a mating notch in the hammer that mates to the trigger's sear surface and they lock together. When the trigger is pulled the sear surface is rotated out of engagement with the hammer and spring tension causes the hammer to rotate and hit the firing pin which in turn strikes the cartridge. Some of the exhaust gasses discharged from firing are routed back through the firearm and push the bolt and bolt carrier backwards and consequently push the hammer down as the bolt and bolt carrier travel rearwards. This happens quickly so the trigger is still depressed at this time. As the hammer attempts to rotate back towards the bolt carrier as it closes, the disconnector catches the hammer to stop the hammer from rotating. As the trigger is released the disconnector disengages from the hammer which resets back onto the trigger. This completes the full cycle operation in semi-automatic mode.
In a full-automatic operation where an auto-sear is used in the firearm, the operator rotates the safety selector to full-auto mode which allows the trigger to move but not the disconnector. The same operation as stated above happens except at the point where the disconnector would normally catch the hammer. At this point the selector is depressing the tail of the disconnector so the disconnector is rotated out of the way. The result is the hammer continues rotating until it hits the auto-sear, which is normally out of the way because the carrier is pushing it rearwardly. The auto-sear then catches the hammer and restricts its movement. Then, as the bolt carrier returns and moves forward to the battery position to pick up another cartridge, the bolt carrier contacts the auto-sear's tail which rotates it out of the way, which thereby moves the auto-sear out of contact with the hammer. The hammer is then allowed to rotate and fire the firearm again. This operation cycle continues until the trigger is released and the trigger's sear surface catches the mating notch in the hammer.